Recently, a method for producing (meth)acrylic acid alkyl ester by reacting a carboxylic acid ester with formaldehyde in the presence of a catalyst (vapor-phase condensation reaction) has been developed. For example, when methyl propanoate is used as carboxylic acid ester, methyl methacrylate is obtained as shown in the following formula (I).CH3—CH2—COOCH3+HCHO→CH3—CH(CH2OH)—COOCH3→CH3—C(CH2)—COOCH3+H2O  (I)
Formaldehyde is used in the form of formalin in many cases. Formalin is an aqueous solution containing formaldehyde, and generally contains methanol as a stabilizer. Therefore, when formalin is used as a raw material of (meth)acrylic acid alkyl ester, water is introduced into the reaction system. When water is present in the reaction system, inhibition of the reaction progression and deterioration of the catalyst are more likely to occur.
A further reaction with an acetal is shown in the following formula (II).R3—CH2—COOR4+R′OCH2OR″→R3—C(:CH2)—COOR4+R′OH+R″OH   (II)
A theoretical example of formula (II) with a dimethoxymethane is shown in the following formula (III).CH3—CH2—COOR4+CH3OCH2OCH3→CH3—C(:CH2)—COOR4+2CH3OH   (III)
The use of dimethoxymethane thus theoretically provides an anhydrous system which avoids the difficulty of subsequent water separation and/or subsequent product hydrolysis. In addition, the use of dimethoxymethane avoids the use of free formaldehyde but nevertheless acts in a general sense as a source of formaldehyde. The absence of water and free formaldehyde could greatly simplify the separation of methyl methacrylate from the product stream.
However, in practice, formula (III) is problematic because methanol dehydrates to dimethyl ether and water. In addition, dimethoxymethane decomposes under catalytic conditions to dimethylether and formaldehyde. Any water formed in these reactions can hydrolyse the ester feedstock or product to its corresponding acid which may be undesirable.
In addition, the presence of water in the reaction mixture increases catalyst decay so that the presence of water may be undesirable even in the production of ethylenically unsaturated carboxylic acids.
Therefore, when (meth)acrylic acid alkyl ester is produced, there is a demand to reduce the amount of water which is introduced to the reaction system, and, for example a method of dehydration by distillation of an aqueous solution of formaldehyde has been proposed (see PTL 1).